This invention relates to portable combustion powered fastener driving tools, and more specifically to a system for varying the power output to such a framing tool.
Portable combustion powered tools for use in driving fasteners into workpieces are described in commonly assigned patents to Nikolich, U.S. Pat. Nos. Re. 32,452; 4,403,722; 4,483,473; 4,483,474; 4,552,162; 5,197,646 and 5,263,439, all of which are incorporated herein by reference. Such combustion powered tools particularly designed for trim applications are disclosed in commonly assigned U.S. Pat. No. 6,016,622, also incorporated by reference herein. Similar condition powered nail and staple driven tools are available from ITWxe2x80x94Paslode under the IMPULSE(copyright) brand.
Such tools incorporate a generally pistol-shaped tool housing enclosing a small internal combustion engine. The engine is powered by a canister of pressurized fuel gas also called a fuel cell. A battery-powered electronic power control unit produces the spark for ignition, and a fan located in the combustion chamber provides for both an efficient combustion within the chamber, and facilitates scavenging, including the exhaust of combustion by-products. The engine includes a reciprocating piston having an elongate, rigid driver blade disposed within a piston chamber of a cylinder body.
A wall of the combustion chamber is axially reciprocable about a valve sleeve and, through a linkage, moves to close the combustion chamber when a workpiece contact element at the end of a nosepiece, or nosepiece assembly, connected to the linkage is pressed against a workpiece. This pressing action also triggers the introduction of a specified volume of fluid fuel into the combustion chamber from the fuel cell.
Upon the pulling of a trigger, which causes the ignition of the gas in the combustion chamber, the piston and the driver blade are shot downward to impact a positioned fastener and drive it into the workpiece. As the piston is driven downward, a displacement volume enclosed in the piston chamber below the piston is forced to exit through one or more exit ports provided at a lower end of the cylinder. After impact, the piston then returns to its original or xe2x80x9creadyxe2x80x9d position through differential gas pressures within the cylinder. Fasteners are fed into the nosepiece barrel from a supply assembly where they are held in a properly positioned orientation for receiving the impact of the driver blade. The fasteners are then propelled through the length of the barrel by the driver blade, exiting the barrel at the workpiece surface. Force of the driver blade and the momentum of the fastener drive the fastener to penetrate the workpiece.
There is considerable shock and vibration that is absorbed by the tool with each firing of the combustion chamber. Rapid movement of the piston within the cylinder due to the expansion of combustion gases and the force of the driver blade on the workpiece tend to propel the tool away from the fastener as it is driven into the workpiece. Immediately following firing of the tool, the hot, expanded gases are purged from the combustion chamber, the combusted gas remaining in the cylinder rapidly contracts, drawing the driver blade back up into the tool within a fraction of a second, tending to recoil and propel the tool in the opposite direction. These forces put large stresses on the housing and all parts of the tool, causing wear where materials flex or parts abrade on each other.
Stresses as described above are particularly acute when short fasteners are driven by the tool. In many framing application, long nails are used predominantly. When driving long nails, more of the force from the power source and exerted through the driver blade is absorbed by the nail as it penetrates the workpiece. As the fastener is driven deeper, additional force is needed to overcome friction between the fastener and the workpiece as the surface area between the two surfaces increases. Short fasteners require less force to completely penetrate the workpiece, so the excess power is absorbed by both the user and the tool. In the extreme, a blank fire, whereby the tool is fired when no fastener is present to absorb any of the shock, puts tremendous stress on the tool, possibly shortening the useful life of the tool.
Control of energy output to a combustion-powered tool is disclosed in U.S. Pat. No. 5,592,580 to Doherty et al., herein incorporated by reference. A voltage divider includes a settable resistance, either a potentiometer or two parallel, fixed resistances that can be alternatively selected, and is used to provide a setpoint voltage. This patent also discloses changing the fan speed in response to light transmission between a phototransmissive diode and a photoreceptive transistor. Thus, it discriminates between fasteners of various lengths, and selected the voltage to the fan depending on the position of the photoelectric switches.
However, reduction in fan speed alone has been unsuccessful in producing a tool that fires consistently at low power. Use of the fan to exhaust the combustion products serves two primary purposes. It produces turbulence in the vicinity of the combustion chamber, promoting heat transfer to cool the tool after firing, as well as mixing of the combustion gases with fresh, oxygenated air. Mere reduction in the fan speed limits both the cooling and replenishment of oxygen in the combustion chamber. When combustion products remain in the combustion chamber in the subsequent combustion cycle, the fuel-to-air ratio may become difficult to control. After several firings, tools running at a low fan speed can have insufficient oxygen to support combustion.
The use of a metering valve to control the flow of fuel into the chamber is disclosed in U.S. Pat. No. 5,752,643 to MacVicar et al. and in U.S. Pat. No. 6,123,241 Walter et al. This invention teaches the use of the metering valve to control the fuel-to-air ratio more precisely to improve the efficiency of combustion. However, use of metering valves with high pressure fluids used in very small quantities are difficult to control.
Thus, there is a need in the art for a power framing tool that is able to efficiently reduce the primary power expended when short nails are in use. There is also a need for a tool that varies the power expenditure automatically, without the need to change settings or switches by the user. In a tool that varies the primary power by changing the fan speed, there is an additional need for an improved system for evacuating the combustion gases following combustion so that they do not build up, interfering with proper fuel to air ratios for efficient combustion.
These and other needs are met or exceeded by the present invention which features an improved system for automatically adjusting the power output of a framing tool based upon the length of the fastener.
More specifically, the invention relates to a combustion power framing tool having a nosepiece for driving fasteners, a housing and a combustion chamber that produces primary power held within the housing. A fastener supply, such as a magazine, is attached to the housing to supply the fasteners. The tool has detects a condition and produces a signal based on that condition, then causes the primary power to vary in relation to the signal and returns to full power following driving of the fastener.
A method includes passing the fasteners past a detector in the tool, detecting the length of the fastener and producing a signal from the detector based on the length of the fastener. After passing the detector, the fasteners are urged through the magazine to a channel and the workpiece contact element is engaged by contact with the workpiece. The primary power varies in relation to the signal when the workpiece contact element is engaged. Combustion of the fuel in the combustion chamber causes driving the fastener into the workpiece at a power level relative to the length of the fastener. Following driving of the fastener, primary power level is returned to full power.
Use of the tool or method described above allows the power of a framing tool to be reduced prior to and during firing of the tool, yet does not allow combustion gases to build up in the combustion chamber. The latter condition makes it difficult to control the air-to-fuel ratio. Under the present method and apparatus, the tool fires consistently and maintains a reasonably consistent power output at least two different power levels. Variation in the speed of the fan provides an easy method of controlling the power from the combustion chamber by varying the power to the fan motor.
Further, the present method and apparatus also automatically adjusts for the length of the fastener. A detector on the tool provides an signal as to the fastener length that is used to vary the power. The tool is saved from wear and tear due to stresses absorbed when small fasteners or blanks are fired. Reduction of power reduces the strain on materials that flex or abrade on each other when fired. Nor is it convenient for the user to have to remember to change a setting or manual lever when changing to a magazine with differently sized fasteners.